1. Field of the Invention
The present invention relates to a brake hydraulic pressure control device for a vehicle using a normally open linear solenoid valve as an inlet valve.
2. Description of the Related Art
Generally, a brake hydraulic pressure control device for a vehicle is known which is arranged between a master cylinder which generates hydraulic pressure according to the stepping force of a driver and a wheel brake which imparts a braking force to a wheel, and controls the braking force using the wheel brake. Such a brake hydraulic pressure control device for a vehicle mainly includes normally open inlet valves which allow transmission of the brake hydraulic pressure to wheel brakes from the master cylinder side, normally closed outlet valves which release the hydraulic pressure (hereinafter also referred to as caliper pressure) within the wheel brakes, and reservoirs which absorb the brake hydraulic pressure released by the opening of the outlet valves. Also, this type of brake hydraulic pressure device for a vehicle is adapted to be able to perform a so-called anti-lock brake control (hereinafter referred to as ABS control) of closing the inlet valves and opening the outlet valves, thereby releasing of the caliper pressure to the reservoirs can be performed to prevent wheel locking, for example, when it is determined that wheels are likely to lock (the slip ratio becomes equal to or greater than a predetermined value).
As such a brake hydraulic pressure control device for a vehicle, conventionally, a brake hydraulic pressure control device adopting as the inlet valves normally open linear solenoid valves (linear solenoid valves) which can arbitrarily change the valve closing force (valve opening amount) according to the power supply amount is known (refer to JP-A-2009-23468). In this conventional technique, as shown in FIGS. 7A to 7C, when the slip ratio (the ratio of wheel speed and vehicle body speed) becomes equal or greater than a predetermined value (time T1), an electric current which has a higher current value Z1 is supplied to the inlet valves which are normally open, the inlet valves are closed at once, and the outlet valves are opened whereby the caliper pressure is reduced. Also, when the slip ratio becomes less than a predetermined value during the ABS control (when the grounding state of the wheels has been restored to a normal state: time T2), the outlet valves are closed, and the electric current to be supplied to the inlet valves is controlled to open the inlet valves with a predetermined valve opening amount (valve closing force). At this time, the power supply amount to the inlet valves is lowered at once up to an initial current value Z2, the power supply amount is reduced at a first gradient toward a turning point current value Z3. Thereafter, the power supply amount is reduced at a second gradient gentler than the first gradient toward a target current value Z4. In this way, by lowering the power supply amount at a comparatively steep gradient up to an electric current (turning point current value Z3) near the target current value Z4, and utilizing braking under a comparatively high caliper pressure for as long a time as possible, a favorable braking force is secured.
Also, in the conventional technique, when the turning point current value Z3 is determined, the turning point current value Z3 is calculated by using a current value Z5 at the time of the end of increasing of the previous increasing cycle (time T1) as the target current value Z4 of the current increasing cycle, and by multiplying a value α1 obtained by subtracting the target current value Z4 from the initial current value Z2 by a predetermined ratio and subtracting a value β1 obtained by this multiplication from the initial current value Z2.
However, in the conventional technique, a current value obtained by subtracting the predetermined ratio (β1) of the variation width (α1) of the electric current assumed this time on the basis of the current value Z5 at the time of the end of previous increasing from the initial current value Z2 is used as the turning point current value Z3. Therefore, when the time interval from the start of increasing to the end of increasing is short (when pressure reduction is low) and long (when pressure reduction is high), there is a problem that the value of the turning point current value may change comparatively greatly, and the feel of the brakes may deteriorate.